Camera systems including an imaging lens and image sensor exhibit a non-uniform image response with a uniform object field input stimulus. Uniformity error is a result of several mechanisms within a camera system including (1) shading following cosine light loss from a lens system across an image field, (2) incident angular dependent pixel response shading in an image sensor, (3) incident angular dependent spectral response of an infrared (IR) filter, (4) spectral or wavelength dependent lens, optical filter, and/or sensor pixel response, and (5) lens-to-sensor misalignment (i.e., pixel micro-lenses with predefined shifts to match lens chief ray angles at an image plane).
Angular and spectral pixel response shading tends to increase as the area dimensions of a pixel decreases and, therefore, correction of non-uniform shading response becomes increasingly important for small image sensor based camera systems. Additionally, lens-to-sensor misalignment tends to require finer tolerances as the pixel size shrinks. Both angular and spectral pixel response shading and lens-sensor misalignment errors lead to the requirement of calibrating each camera module after assembly to restore a uniform output response. This technique of camera shading calibration is referred to herein as module-level shading calibration (MLSC).
An MLSC is generally performed during production of camera modules such that each camera undergoes a flatfield illumination test. Each camera in the production line is exposed to a uniform flatfield illumination source to determine shading calibration values required for the module under test. An MLSC using a single illuminant (fixed spectral response and color temperature) may effectively calibrate shading associated with lens-sensor misalignment, pixel shading, and lens shading.
An MLSC using a single illuminant, A, may not effectively calibrate spectral shading variation that results from illuminating a flatfield scene with a different color temperature illumination source, B, while applying shading calibration values found using illumination source A. Spectral shading variation, or color shading, may appear as a visible artifact in images captured under illumination sources that are different than the production calibration source.
Multiple illumination sources with desired spectral characteristics that match real world illuminants may be used during production in order to find shading calibration values for each expected real world illuminant. The camera may use an illuminant detection algorithm, such as auto white balancing, to select an appropriate shading calibration for use on the current scene.
One difficulty with multiple illuminant MLSCs is the cost increase in the production of camera modules. For example, if five illuminants are required to calibrate spectral shading variation for all real world illumination scenarios, then the production system may require five uniform illumination sources or spectral shift filters. In addition, five flatfield images may be required for capture and stored per camera module to gather sufficient calibration values. Cost increases because of the physical equipment required by the production test system and the increased time spent per camera to capture data. There is a desire, therefore, to perform multiple illuminant MLSCs to correct shading errors and not increase production cost.